1. Field of the Invention
This invention relates to an epoxy resin composition having good heat resistance of solder and further having excellent reliability.
2. Description of the Prior Art
Epoxy resins have excellent heat resistance, moisture resistance, electrical characteristics and adhesion properties, and they can acquire various characteristics on modifying the recipes thereof. Accordingly, therefore, epoxy resins are used in paints, adhesives, and industrial materials such as electrically insulating materials.
As methods of encapsulating electronic circuit parts such as semiconductor devices, there have been proposed a hermetic encapsulating method using metals or ceramics, and a resin encapsulating method using phenolic resin, silicone resin, epoxy resin or the like. From the view point of balancing economy, productivity and physical properties, however, the resin encapsulating method using an epoxy resin is mainly adopted.
On the other hand, integration and automated processing have recently been promoted in the step of mounting parts to a circuit board, and a "surface mounting method" in which a semiconductor device is soldered to the surface of a board has been frequently employed in place of the conventional "insertion mounting method" in which lead pins are inserted into holes of a board. Packages are correspondingly in a transient stage of from conventional dual inline package (DIP) to thin-type flat plastic package (FPP) suitable for integrated mounting and surface mounting.
As with the transition to the surface mounting method, the soldering process which conventionally has not attracted attention has now come to be a serious problem. According to the conventional pin insertion-mounting method, only a lead part is partially heated during soldering, whereas according to the surface mounting method a package in its entirety is dipped and heated in a heated solvent. As the soldering method for the surface mounting method, there are used solder-bath dipping method, solder reflow method in which heating is carried out with inert-liquid saturated vapor and infrared ray, and the like. By any of the methods, a package in its entirety is to be heated at a high temperature of 210.degree. to 270.degree. C. Accordingly, in a package encapsulated with a conventional encapsulating resin, a problematic cracking of the resin portion occurs at the soldering step, whereby the reliability is lost, and hence, the obtained product cannot be practically used.
The occurrence of cracking during the soldering process is regarded due to the explosive vaporization and expansion, at heating for soldering, of the moisture absorbed in the time period from procuring to the mounting process. For the countermeasure, there is employed a method to completely dry up a post-cured package and enclose it in a hermetically sealed container for shipping.
The improvement of encapsulating resins has been investigated in a wide variety of ways. For example, heat resistance of solder can be improved by a method of adding an epoxy resin having a biphenyl skeleton and a rubber component (Japanese Unexamined Patent Publication No. 251419/1988), but it is not sufficient. The method of adding an epoxy resin having a biphenyl skeleton and microparticles in powder of a particle diameter less than 14 .mu.m (Japanese Unexamined Patent Publication No. 87616/1989) does not yield a satisfactory level of heat resistance of solder.
Alternatively, there has been proposed the addition of spherical fused silica microparticles (Japanese Unexamined Patent Publication No. 263131/1989), whereby only the fluidity of encapsulating resins is improved and the heat resistance of solder is not sufficient.